Wednesday, March 16, 2011

Inferring the Upper-Air Pattern from a Single Sentence

In reviewing all the weather blogs I read this morning, I noticed this line in WGN-Chicago's Weather Blog:

"...Powerful southwest winds predicted to gust to 35 mph Thursday afternoon combined with jet stream-induced warming produced as air sinks and compresses beneath the nose of powerhouse 170 mph upper winds should more than compensate for the reduction of sunshine and allow warming to proceed."
--WGN Weather Blog, March 16, 2011.

Hmm...this looks like something we can explore with our knowledge of jet streak dynamics.

Assuming the statement above is correct (and it was written by Tom Skilling, who is one of my favorite and most trusted TV meteorologists, so I have no doubt that it is correct...), we can start making some inferences about what the weather pattern must look like without even looking at a map and just using our knowledge of what dynamics are necessary to produce the kinds of effects described.

First, we can consider the statement about the jet-stream winds. Tom Skilling states that air will be "sinking" beneath the "nose" of the upper-level winds. Now, the "nose" of a jet is just a colloquial term used in the meteorology community to refer to the exit region of the jet streak--the leading edge of it on a map. Since he's talking about sinking motion under the exit region (the "nose") of the jet, we're looking for some jet streak structure that allows for sinking motion under the exit region. Convergence aloft leads to sinking motion throughout the lower atmosphere, so we're looking for a jet streak pattern that has convergence in the exit region. We can consider curved jet streaks or straight jet streaks.

We saw that in cyclonically curved jet streaks (like on the lower-left part of the above image) there was convergence in the entrance region and divergence in the exit region. Well, that's not what's being described above--we wanted convergence in the exit region since the motion below it is described as "sinking". However, in the anti-cyclonically curved jet streak above (the upper-right side), we see that in such a jet streak, we do expect convergence in the exit region of the jet. So here's one possibility--there could be an anti-cyclonically curved jet aloft.

But what about a straight jet streak? Here's a schematic of divergence and convergence associated with a straight jet streak:

We see in the diagram above that it is possible to have convergence in the exit region of a straight jet streak, albeit the right exit region. There would be implied divergence in the left exit region. But still--if the jet streak were to the north of Illinois, Chicago would still be under the convergent region of the jet and we'd see sinking motion and warming like the description above says. So, this too is a possibility.

So which is it? Are we expecting a straight or a curved jet streak? Well, we can also infer that the jet streak must be rather straight. How? By the first part of the statement above:

"...Powerful southwest winds expected to gust to 35 mph..."

That's a description of what's going on at the surface. So we're expecting strong winds. How do we get strong winds at the surface? We need a low-pressure center at the surface somewhere to provide the pressure gradient necessary to generate such strong winds. Where would this low-pressure center be? The wind direction tells us that. Remember that winds tend to blow roughly counter-clockwise (cyclonically) around a low pressure center. Therefore, to get winds out of the southwest, the low pressure center must be off to the north or northwest somewhere.

So we'd expect a surface low to be someplace off to the north-northwest or so. It's also probably a pretty deep low to be generating 35 mph winds and it also can't be too far away. But to support such a strong low-pressure center at the surface, what must be happening aloft? We have to be seeing divergence aloft--it's the same argument we always would use for finding areas of strong vertical motion underneath jets. Divergence aloft leads to rising motion which in turn causes the pressure at the surface to fall as all that air is lifting away.

So now we can tie this back into our upper air pattern. We see our jet aloft most be able to satisfy two conditions to fit this story:

There has to be a region of convergence aloft over northern Illinois to be producing the downward motion (and warming) that are being advertised.

There has to be a region of divergence aloft somewhere to the north or northwest of Illinois to support a surface low-pressure center off to the north or northwest. We know there is probably a low-pressure center there because the winds in northern Illinois are forecast to be strong and out of the southwest.

An anti-cyclonically curved jet streak doesn't really fit the bill--it doesn't really allow for an area of divergence off to the north or northwest. However, a straight jet streak does allow this--we can position a straight jet streak just north of Illinois to satisfy all of these criteria:

So now we have a good guess as to what the upper-air pattern (and, also from this line of reasoning, the surface pressure map as well) should roughly look like.

So how does this compare to the models? Here's the GFS forecast for mid-day Thursday up at 250mb:

Not too bad at all for a rough guess. The jet streak itself is rather straight with a slight anti-cyclonic bend to it. The shading can kind of throw you off when trying to find entrance and exit regions. If we just shaded the stronger wind speeds in the core of the jet (like, the yellows, oranges and reds in the above image) we do see a broad exit region of the jet extending through the Dakotas and into Minnesota. Because this jet doesn't seem to have a very focused exit region in the model, we'd expect the surface pressures related to it to also be kind of broad. Here's the GFS forecast for the surface at that time:

It's a very broad area of low pressure, but it is generally located to the north-northwest of Illinois like we expected. Also the winds are strong out of the southwest, accordingly. It all agrees rather well.

It's pretty fun to be able to pull together a full synoptic pattern from just that one sentence...

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
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